1. Field of the Invention
This invention relates to drilling and producing wells underwater and particularly to supporting a marine riser extending upwardly from the bottom of a body of water.
2. Description of the Prior Art
In recent years the search for oil and gas has extended into increasingly deeper waters. Economic considerations and physical limitations frequently militate against the use of bottom supported platforms in very deep water. Therefore, most offshore drilling and production in deep water is conducted from a floating drilling or production platform which supports the drill rig and derrick and associated drilling equipment and/or production equipment. A marine riser is normally used to interconnect the floating platform and the subsea equipment such as a wellhead located upon the seafloor.
A marine riser may be employed in offshore operations to (1) guide tools and components into a well being drilled and to circulate drilling fluids and cuttings; (2) to convey fluids and tools from a floating vessel and a subsea installation, (i.e., subsea well, template, manifold, etc.) The marine riser is presently regarded as the limiting element in floating drilling operations and/or production operations since the weight of the marine riser and the stresses within the riser increase with water depth. Adding to the stress on the marine riser are bending moments caused by the action of wind, wave and sea currents on the riser and by movements of the floating platform.
To counteract marine riser stress, riser tensioning devices are normally mounted on the floating drilling or production platform. These tensioning devices apply a tensile force to the top of the marine riser, thereby reducing bending stresses on the riser. The use of flexible joints placed at the ends of the riser has also been used to increase riser flexibility. However, both riser tensioning devices and flexible joints have limitations as to the amount of riser stress which they can relieve.
Riser tensioning systems may be divided into "active" and "passive" systems. Active riser tensioning systems using hydraulically-driven piston and cylinder tensioners or other means (i.e., elastomeric springs) are subject to mechanical failure. Such failure may cause the marine riser assembly to collapse, severely damaging the subsea equipment and/or causing an underwater blowout with subsequent pollution and risk of danger to the floating platform, its equipment and crew. As noted in other patents, a passive riser tensioning system may be employed to avoid the problems of an active riser tensioning system. One form of a passive riser tensioning system comprises at least one buoyant member, such as a buoyancy chamber, or shaped foam floatation sections attached near the upper end of the marine riser in order to apply upward buoyant force to the riser. Use of such buoyant members decreases the amount of tensioner equipment that must be carried by the floating platform and eliminates tension reaction forces from the vessel thus reducing the vessel's buoyancy requirements.
As shown for example, in U.S. Pat. No. 3,017,934 entitled "Casing Support" issued Jan. 23, 1962, to A. D. Rhodes et al, a series of buoyant members may be attached around the periphery of the marine riser in order to supply the upward buoyancy necessary to apply tension to the riser. As shown in patent '934 however, the buoyancy members or tanks terminate some distance below the floating platform. Maintenance on these buoyancy members requires that the marine riser be disconnected from the subsea equipment, or that underwater operations be conducted in order to repair and/or inspect the submerged members. Each buoyant member must also be attached in a tedious manner to the riser as the riser is being assembled and lowered downwardly to the subsea equipment.
Each time that it becomes necessary to inspect one of these buoyant members, production from the subsea equipment must be interrupted and the riser disconnected from the subsea equipment in order to allow the riser buoyant member(s) to be removed to the surface. Accidental or emergency disconnection of the riser from the subsea equipment would expose the floating platform positioned above the riser to damage from the upwardly surging riser, with attendant risk to the crew of the platform.
A method and apparatus therefore need be developed that allows a riser to be passively tensioned by a buoyant member, without the inherent disadvantages mentioned previously in the use of such a member. A method and apparatus need be developed that permits maintenance operations on such a member without disrupting normal use of the marine riser. Such a member must be capable of easy removal from the riser in order to simplify the removal and/or inspection process of the member. The member should also be capable of easy attachment to the riser as the riser is assembled and/or connected to the subsea equipment.